I co-slept with our 3 1/2 year old as well. It worked out fine for us because my husband works the night shift and I liked having him in my room much better because of being alone in the house. When I got pg with our 2nd son (born June 3) I thought it was time to move him to his own bed. I had the same issue, it worked out ok at first, then he saw that someone was taking his place and wanted back. We bought him a new bed and everything. The only thing that seemed to work was Mason crying and waking him up in the middle of the night. Even that caused issues because I was breastfeeding and he was irritated because I had to pay attention to the baby instead of him when he was upset. It just took some time, and patience. I explained to him why it was better to sleep in his own bed. It is a hard transition, and I cried almost as much as he did. lol But sometimes the best thing you can do is be the "mean momma" and make her go to her own bed.
What I did was start a new routine for him, our own special night routine that could only be done in his room. I let him pick out a book (we had gone out a bought a bunch of new books) and we would read it each night. Then we sang songs that he got to make up words for. I would start the song, then he would throw something in. We discussed the day, what he liked best about that day and what he would like to do tommorow. If he would want to go to my room for bed I would explain to him that because Mason was in there that if we went in there I would not be able to sing, talk, or read. He would still come to my bed in the middle of the night, and I admit sometimes when I was just so tired it was easier to let him stay, but most of the time I would hug him for a minute, let him get some mommy time and then we would go back to his bed. Now it's kind of funny because even if I want him to come to my room (like nights when it is storming bad or during the hurricanes) he is like NO WAY. lol
Oh yeah I forgot, we also bought him a special little flat screen tv that we hung on the wall. We would also have "tv" night where I would put an educational DVD in and we would sit and watch it and discuss what was going on. I have these short DVDs that are only about 30 minutes so I do not have to cut it off half way through. Although I am sure there are moms out there that are totally against tv time for his age, I found that he has learned so much from these shows, with my interaction.
HTH and good luck with your little ones.
T.